1. Field of the Invention
The present invention relates to a method for fabricating an MOS semiconductor device, and particularly to a method for fabricating an MOS transistor having the LDD (Lightly Doped Drain/Source) structure.
2. Description of the Related Art
As integrated circuit technology progresses to achieve a higher performance and a higher integration density, the requirement on the MOSFET device becomes more demanding. In particular, phenomena such as lowering in breakdown voltage, hot electron effects, and short-channel effects, which result from high-field intensities in a channel region, limit the minimum channel length and maximum operating voltages. Under the above circumstance, a self-aligned, Lightly Doped Drain Field-Effect Transistor (LDDFET) has been proposed.
According to this technique, narrow N.sup.- regions are added between a channel region and N.sup.+ drain and source regions so that a peak electric field in the channel is reduced, resulting in a significant increase in the allowable operating voltage, and a reduction in gate-source/drain capacitance.
The LDD type MOS device is conventionally fabricated as follows. First, a gate electrode is formed on an active region of a P-type surrounded by a field oxide layer via a gate insulating layer. Then, N-type impurities such as phosphorus are lightly implanted into a surface of the active region to form shallow N.sup.- regions. Subsequently, a masking layer is selectively formed above the channel region and its peripheral regions of the shallow N.sup.- regions, and then N-type impurities such as boron are introduced into the surface of the active region to form deep source and drain contact regions. However, in the conventional method, it is difficult to form the shallow regions at high impurity concentration and therefore sheet resistance of the shallow regions is high. Therefore, effective resistances of the drain and source regions are large and current amount flowing therethrough is limited. Even if the shallow regions of high impurity concentration is formed, the depletion layers from such shallow regions extend widely to cause so-called short channel effect. Moreover, it is difficult to form the masking layer precisely, because the masking layer is formed by independent mask-align procedure. Thus, it is difficult to form the shallow regions in the precise relationship with the gate and the drain/source regions.